In present day computing systems a high demand has emerged for increased data storage capability in physically lighter and smaller mass memory storage devices. Magnetic media disk memories have necessarily become lighter and smaller, while at the same time becoming capable of storing more data than their physically larger predecessors.
In general, disk memories are characterized by the use of one or more magnetic media disks stacked on a spindle assembly and rotated at a high rate of speed. As requirements have emerged for these magnetic disks to be capable of more storage with less size and weight, the spacings and tolerances involved in disk recording systems have become exceedingly minute. As a result, the most important properties needed in advanced magnetic media disk memories are now generally of a mechanical nature.
The mechanical texturizing process of the surfaces of the disks, performed during disk manufacture, has become an increasingly delicate and exacting process. Most texturizing equipment utilizes an abrasive material, such as silicon carbide or aluminum oxide, for cutting small grooves in the disk, which is commonly made of aluminum. The material is typically bonded to a mylar-backed tape which is then passed over a cylindrical load roller. The tape is mechanically forced against the surface of the disk by the load roller. Commonly, two load roller assemblies are positioned side by side to texture the front and back surfaces simultaneously. To facilitate the texturizing process, the rigid-disk substrate is often rotated against the tape/roller system at a high rate of speed.
Assemblies which repetitively mount, hold, and rotate the rigid-disk substrates typically comprise a robot arm and a collet system. The robot arm seizes an untexturized disk from a disk source and positions the inner surface of the disk over the closed fingers of the collet system. An expander arm is then axially forced against the inside of the collet fingers, causing them to open and to thereby radially clamp the inner surface of the disk.
The disk is held in place as a result of the radial pressure exerted by the outer surfaces of the collet fingers against the disk's inner surface. This radial pressure, together with the resultant static friction at the contact points between the collet fingers and the inner surface of the disk, holds the disk in place while being rotated for texturization. This holding/rotating force overcomes the kinetic friction between the disk surface and the texturizing tape, which resists the rotation. This kinetic friction is a function of the (1) kinetic friction coefficient between the texturizing tape and the disk surface, and (2) the orthogonal force of the tape against the disk surface, which is typically about 5 pounds for a 3.5" disk, exerted simultaneously on the front and back sides of the disk.
After texturization, the expander arm is axially released, closing the collet fingers to allow removal of the finished disk and mounting of the next disk for texturizing.
Due to the ever-present demand for smaller and lighter storage units, the thicknesses of the rigid disk substrates have decreased. For example, whereas a 3.5" diameter, 500MB disk used to have a thickness of about 0.050", a disk with the same diameter and capacity is now typically less than 0.025" thick. These disks, however, need at least the same degree of precision texturizing and flatness as their thicker predecessors and therefore must be subject to the mechanical texturizing process described above.
Problems have arisen regarding the collet systems which mount, hold, and rotate the increasingly thin disk substrates. These problems have arisen in light of the following: (1) the thinner disks are less rigid and are therefore subject to significant warpage if there is sufficient clamping force exerted against their inner circumferences, (2) among a sample of disk substrates, there will be small but crucial variations in the inner diameter of the disks, and (3) these variations may result in less surface area along a given disk's inner circumference to which the collet can radially clamp. Current collet systems have deficiencies in light of the above problems brought on or exacerbated by the increasing thinness of the disks, deficiencies which will be described with reference to a typical prior art system.
FIG. 1 diagrams a disk clamping collet system 100 according to the prior art. Disk clamping collet system 100 comprises a collet 102, an expander 104, a draw bar 106, a spindle 108, and a spring means 110.
Collet 102 comprises a base 112 and fingers 114a, 114b, 114c, 114d, 114e, and 114f. Each of fingers 114a-f is a radially curved cantilever extending longitudinally from the base 112 and is integral with the base 112. Each of fingers 114a-f is radially curved around the axis of rotation of the collet system 100, this axis being designated the z-axis in FIG. 1. Each of the fingers 114a-f is curved approximately 60 degrees around the z-axis and is separated from its neighboring fingers by a radially oriented slot running along the z-axis. While the collet 102 in FIG. 1 has six fingers 114a-f, in general there may be N fingers, in which case the arc covered by each finger around the z-axis will be approximately 360/N degrees. Fingers 114a-f are substantially identical to each other. Collectively when viewed as a whole, fingers 114a-f form a finger portion 116 of collet 102.
Finger 114a of collet 102 comprises an expansion portion 118a formed therein by a surface which slopes inwardly toward the base 112 and z-axis, as shown in FIG. 1. Fingers 114b-f likewise comprise expansion portions 118b-f, respectively. Collectively when viewed as a whole, expansion portions 118a-f form an expansion portion 120 of collet 102.
Finger 114a of collet 102 further comprises a disk contact ring segment 122a formed thereon by a raised surface, the longitudinal dimension of which corresponds generally to the thickness of the disk to be mounted. Fingers 114b-f likewise comprise disk contact ring segments 122b-f, respectively. Collectively when viewed as a whole, disk contact ring segments 122b-f form a disk contact ring 124 of collet 122.
Disk clamping collet system 100 also comprises an expander 104. Expander 104 lies at least partially within a hollowed portion of the collet 102 formed by the longitudinally extending fingers 114a-f and comprises an expansion surface 126. Expansion surface 126 has a conical shape which generally forms a counterpart to the conical shape of the expansion portion 120 of collet 102. When expander 104 is urged in an axial direction toward the base 112 of collet 102, contact is made between the expansion surface 126 and the expanding portion 120 such that fingers 114a-f are each urged radially outward in a cantilever fashion. Disk contact ring 124 thus obtains a larger diameter and comes into contact with the inner circumference of the disk, thereby holding the disk in place.
Disk clamping collet system 100 further comprises a draw bar 106 which is axially affixed to expander 104 such that expander 104 is moved axially in response to movement of the draw bar 106. Disk clamping collet system 100 further comprises a spindle 108 which lies stationary with respect to the z-axis of FIG. 1, while collet 102 is axially affixed with respect to spindle 108. In this way, draw bar 106 and expander 104 are axially moved along the z-axis while collet 102 and spindle 108 are stationary along the z-axis of FIG. 1. The entire system 100 rotates around the z-axis.
Draw bar 106 comprises an abutting end 128, as shown in FIG. 1, wherein spring means 110 is positioned between abutting end 128 and spindle 108. In the prior art device of FIG. 1, spring means 110 comprises a stack of belville springs having a high spring constant and small displacement range. Spring means 110 urges abutting end 128 of draw bar 106 away from spindle 108, which in turn urges expansion surface 126 of expander 104 into contact with expansion portion 120 of collet 102.
In operation, during disk unloading/loading, abutting end 128 of draw bar 106 is forced toward spindle 108 by an external means such as an air cylinder assembly (not shown). Expander 104 thus moves outward with respect to collet base 112, reducing the outward radial pressure exerted by expansion surface 126 on expansion portion 120, and in turn reducing the outward radial force exerted by disk contact ring 124 on the mounted disk. The disk can then be removed and another disk positioned over the disk contact ring 124. After this step, the external force on abutting end 128 is released and the new disk thereby clamped into place by (1) operation of the spring means 128 against draw bar 106, causing (2) outward radial pressure of expansion surface 126 against expansion portion 120, causing (3) disk contact ring 124 to expand outwardly against the inner surface of the disk.
The range of motion of the draw bar 106 and expander 104 with respect to the collet 102, known as the "stroke length" of the collet system, is quite small, equalling only about 1/32 of an inch in a typical system according to the prior art as shown in FIG. 1.
Problems have arisen with the prior art system according to FIG. 1 due to the decreasing thickness of the disk substrates. When thick disks are mounted onto disk contact ring 124, fluctuations from disk to disk in the radial force exerted by disk contact ring 124 are not as significant because the thicker disks are capable of maintaining their shape even when this force exceeds nominal parameters. However, thinner disks will tend to warp as a function of the asymmetrical outward radial pressure. The degree of warpage depends on the magnitude of the radial pressure. At the same time, however, the thinner disks need just as much rotational holding power as the thicker disks because the texturizing force exerted on the disk's front and back surfaces remains generally the same. A greater outward pressure is thus needed on the inner circumference of the disk because the area along the inner circumference is smaller because the disk is thinner.
Because of these constraints, it is desirable to clamp the inner circumferences of the disks with symmetrical clamping forces which are high but which do not exceed parameters beyond which the disk will warp excessively. Such characteristics will be increasingly crucial as disk thicknesses continue to decrease. It is thus desirable that a disk clamping collet system be able to exert a highly repeatable outward force on the inner circumference of successively mounted disks. It is further desirable to make this force more tolerant to variations in the inner diameters of the successive disks.
The prior art disk clamping collet systems are deficient with respect to these desired criteria, as is shown graphically in FIG. 2. FIG. 2 plots the outward force F.sub.out exerted by one of the fingers 114a-f (e.g., finger 114a) against a portion of the inner circumference of a disk which has been placed over disk contacting ring 124. This force F.sub.out, shown on the vertical axis, is plotted against the axial displacement x of draw bar 106 with respect to the spindle 108, and therefore which also represents the axial displacement of expander 104 with respect to collet 102, as shown in FIG. 1.
As shown in FIG. 2, when displacement x is below a nominal clamping displacement x.sub.o, there is no contact between the disk contact ring segments 122a-f of fingers 114a-f and the disk. As x is increased, i.e. as the draw bar 106 is further urged outward by the spring means 110, thereby urging expander 104 further into collet 102, there is finally contact as the fingers 114a-f meet the disk at the nominal clamping displacement x.sub.o. At this nominal clamping displacement x.sub.o, the force F.sub.out against the disk is at its nominal operating level F.sub.out,o. If the distance x is increased beyond this point, however, the force F.sub.out increases dramatically, and thus a slight translation of the expander 104 at this operating point will easily cause forces which will result in warpage or buckling of the disk, which is undesired.
This high sensitivity to axial translation of the expander 104 is exacerbated by another problem: the discontinuous relationship between the displacement x and the axial spring force exerted by the spring means 110 when x is near the operating point x.sub.o. This brought about by friction problems which exist at the contact points between the expansion surface 126 and the expansion portion 120 and changes in the inner diameter of the disk due to manufacturing tolerances. Near the operating point x.sub.o, the nature of the contact achieved between the expansion surface 126 and the expansion portion 120 tends to vary back and forth between a static and a kinetic relationship, with the friction coefficient experiencing accordingly erratic behavior. As a result, there is a discontinuous, highly nonrepeatable cause-and-effect relationship between the axial spring force exerted by spring means 110 and the displacement x resulting therefrom.
The variations among the static and kinetic friction coefficients is accompanied by the further problem of an abundance of abrasive elements which exist in free floating form in the texturizing environment as a result of, for example, the abrasive materials becoming dislocated from the mylar-backed abrasive tape during the texturizing process. These abrasives can accumulate on surfaces and cause changes in both static and kinetic friction coefficients among the various parts of the disk clamping collet system 100, including the expansion surface 126 and the expansion portion 120.
A further disadvantage associated with the prior art disk clamping collet systems is their extreme sensitivity to variations among different expanders 104. As illustrated in FIG. 3, an exploded view of expansion surface 126 and expansion 120 of collet 102 is shown. As described above, expansion surface 126 has a conical shape which cooperates with the conical shape of the expansion potion 120 of collet 102. Because of manufacturing tolerances, the slope of the expansion surface 126 tends to vary slightly among different expanders 104. A slight variation in the slope of expansion surface 126 greatly affects the point of contact x between the expansion surface 126 and the expansion portion 120. As discussed above in connection with FIG. 2, translation of x from the nominal clamping displacement x.sub.o will cause forces which will result in excessive warpage or buckling of the disk. Therefore, these clamping systems are highly sensitive to replacement of one expander for another expander, thereby adversely affecting repeatability.
Thus, there exists in the prior art system the problems of (1) high sensitivity of the outward radial force exerted by the collet fingers on the disk to small axial displacement changes of the expander, and (2) a discontinuous, nonrepeatable relationship between the axial displacement of the expander and the axial spring force exerted thereon. The result of these deficiencies is that the clamping force exerted on a mounted disk is inconsistent and non-repeatable. As disks get thinner, it will be desirable to achieve more exacting, repeatable outward pressures which sufficiently clamp but do not warp the disk, and therefore systems according to the prior art of FIG. 1 are insufficient.
FIG. 4 shows one prior art attempt to solve to problems of the disk clamping collet system of FIG. 1. The modified system of FIG. 4 employs a mechanical outer-limit stopping means to prevent over-expansion of the disk contact ring 124, in the form of a collar 360 fitted over the collet 102. The collar 360 prevents the outward expansion of disk contact ring 124 beyond a certain point and thus will normally prevent warpage of the disk.
However, the disk clamping collet system of FIG. 4 has a drawback in that small fluctuations in the inner diameter of the disk among a sample of disks will cause serious problems, especially where the disk has a slightly larger inside diameter than normal. In the case of a larger-than-normal inside diameter disk, the collar 360 will prevent the collet 102 from expanding outwardly to a sufficient clamping diameter, causing the disk to slip during texturizing due to insufficient clamping force. When the disk is slightly smaller than normal, the collar 360 will prevent the collet 102 from expanding beyond a certain point. However, this stopping point may be beyond a point where the disk has already warped. Thus, the prior art device as shown in FIG. 4 is intolerant to variations in the inner diameters of successive disks.
It is an therefore an object of the present invention to provide an improved disk clamping collet system which provides for a more constant, repeatable outward radial force on a mounted disk during the disk texturizing process.
It is a further object of the present invention to provide a disk clamping collet system which is more tolerant to variations in inner disk diameters among a sample of nominally identical disks.
It is yet another object of the invention to provide a disk clamping collet system which is more tolerant to the presence of free floating abrasives in the disk texturizing environment.
It is still another object of the invention to provide for greater coupling force to rotate a mounted disk against the resistance of the texturizing abrasives, while avoiding warpage of the disk due to excessive outward radial forces on its inner circumference.
It is still another object of the present invention to provide for an improved disk clamping collet system which allows easier mounting of the disk by allowing for a greater margin of error in the centering of the disk as the disk is mounted by an external positioning mechanism.